Production of Blade Runner was often done in a rush, so visual effects directors sometimes had to make do with pre-existing props. Paul M. Sammon writes in Future Noir:

Amusingly, another very distinctive spacecraft also found its way into this part of the cityscape. "Bill George had been making a replica of the Millennium Falcon, Han Solo's ship from the Star Wars movies, for his own amusement," Stetson explained. "It was about five feet tall. At the time we were so frantic to get more buildings into the cityscape that we grabbed Bill's ship, bristled it with etched brass, and plopped it into different shots. Instant building.

The Millennium Falcon model has been outlined in the above image. You can see other, more hazy images of her at the link.